The present invention relates to a thin film oxygen sensor incorporating an oxide semiconductor and having a structure wherein a sensor element is integrally formed with a heater element.
An exhaust gas cleaning system has been known as a method for improving fuel consumption of an internal combustion engine of an automobile and for decreasing toxic components in an exhaust gas, wherein the concentration of oxygen contained in the exhaust gas is detected, and the air or fuel quantity is controlled in accordance with the detection signal. In particular, the present invention relates to an improvement in an oxygen sensor (especially, of a thin film oxygen sensor of an oxide semiconductor) used for such an exhaust gas cleaning system.
Conventional problems are described hereinafter. Oxide elements such as TiO.sub.2, CoO, CeO.sub.2, Nb.sub.2 O.sub.5 and SnO.sub.2 are used for oxide semiconductor type ceramic oxygen sensors (of resistance change type) which carry a catalyst such as Pt. When the characteristics of these oxygen sensors are actually examined by being placed in an exhaust gas from an internal combustion engine, and changes in resistances of the senors are measured when the air-fuel ratio changes, the characteristics of such changes in resistance are greatly different in the following cases: (1) when the engine is started; (2) when the exhaust gas is warmed; and (3) when the exhaust gas is sufficiently heated to a high temperature. The operation state of the internal combustion engine varies; the engine is operated at random in a deceleration, acceleration or constant speed mode. For this reason, the temperature and flow rate of the exhaust gas greatly vary, so that the components of the exhaust gas reaching the oxygen sensor greatly vary.
(a) Application of Oxygen Sensor
A resistance-change type oxygen sensor (resistance R.sub.2) using an oxide semiconductor is connected in series with a reference resistor (resistance R.sub.1). A voltage V.sub.1 is applied from a constant voltage source to the series circuit of the oxygen sensor and the reference resistor. An output voltage V.sub.2 appearing across two ends of the sensor having the resistance R.sub.2 is given as V.sub.2 ={R.sub.2 /(R.sub.1 +R.sub.2)}V.sub.1 in accordance with a change in the sensor resistance R.sub.2. The output voltage V.sub.2 is entered as data in an air-fuel ratio control computer and is compared with a reference voltage. The computer determines that the air-fuel ratio represents a "lean" (insufficience fuel) state when the output voltage is higher than the reference voltage. Otherwise, the computer determines that the air-fuel ratio represents a "rich" (excessive fuel) state. The fuel injection quantity is controlled in accordance with the determination result so as to always combust the fuel at the theoretical air-fuel ratio, thereby improving combustion efficiency.
The following methods are considered to improve precision in controlling the air-fuel ratio of the resistance-change type oxygen sensor:
(1) A thermistor is inserted in series with the sensor to compensate for temperature dependence. PA0 (2) The sensor is heated to and kept at a predetermined temperature to decrease temperature dependency of control precision. PA0 (3) Since the resistance of the oxide semiconductor greatly changes in accordance with an oxygen partial pressure, a catalyst is carried on the surface of and inside the sensor so as to completely combust a noncombusted gas, thereby greatly changing the oxygen partial pressure in the vicinity of the sensor.
(b) Response Control
In order to accurately detect a change in the air-fuel ratio of the engine, a thin film method unlike the conventional sintering method was used by the present inventors to manufacture a sensor in such a way as to improve the response characteristics of the sensor. As a result, the thin film sensor had a high speed response about 5 to 10 times that of the conventional sintered sensor.
When the sensor was mounted in an automobile driven in a test run, it was found that no problems occurred provided the engine speed was not less than 4,000 [rpm], but that controllability of the engine was degraded when the engine was operated at a low speed. This is caused by the fact that any slight difference between the cylinder injectors which control the fuel injection quantity results in variation in the air-fuel ratios of fuel quantities injected into the respective cylinders at a low speed of not more than 1,000 [rpm]. For example, when the quantity of fuel injected into the first cylinder differs from that injected into the second cylinder, the air-fuel ratio with respect to the first cylinder differs from that of the second cylinder since the quantity of air supplied to the first cylinder is the same as that supplied to the second cylinder. As a result, this difference is detected as a change in the air-fuel ratio by the sensor disposed in the common portion of the intake manifold, and the sensor having high speed response detects it as the variation of the air fuel ratio thus resulting in degradation of controllability.
In general, the sensor must have good sensitivity characteristics for accurately detecting a change in air-fuel ratio. However, when the response characteristics of the sensor are improved, it has an increased tendencey to be influenced by a distrubing signal, resulting in erroneous operation. Therefore, the response characteristics of the sensor must have a proper response range. An engine control sensor preferably has a response range from about 50 msec to 1000 msec.
(c) Cause of Poor Reproducibility and Low Stability
The present inventors examined why a resistance-change type oxygen sensor incorporating an oxide semiconductor had low stability and poor reproducibility.
A sintered TiO.sub.2 sensor was mounted in an automobile which was subjected to a test run. This sensor was heated to and kept at by a built-in sensor a temperature of 700.degree. C. to 800.degree. C. and had a sufficient catalyst to increase activity.
The following phenomenon occasionally occurs when an engine is started and operated at a steady state and changes in resistance of the sensor are examined. Even if the resistance of the sensor is very small and the air-fuel ratio is given as lean, the sensor detects a similar result as if the air-fuel ratio of the engine was changed to a rich ratio.
It was found that noncombusted components such as soot or tar become attached to the surface of the sensor when the air-fuel ratio was set to a richer ratio (excess air factor .lambda.=0.5 or less), thereby impairing stability and reproducibility. Further, even if the catalyst of the sensor is highly active, the gasoline cannot be completely vaporized, and can only be partially combusted at the start of engine operation. As a result, an unacceptably rich state can be obtained. Furthermore, since the temperature of the exhaust gas is low, incomplete combustion occurs. Therefore, even if the sensor is kept at a predetermined temperature, the noncombusted components become attached to the surface of the sensor when the sensor is exposed in a rich atmosphere. Although the noncombusted components attached on the surface of the sensor can be partially combusted when the sensor is heated, the noncombusted components may not be completely combusted. When some of the noncombusted components are left on the sensor, such residual components have a higher conductivity than TiO.sub.2, which constitutes the sintered sensor. As a result, even if the resistance between sensor terminals is low, a similar signal is generated as if the rich atmosphere obtains.
In the conventional resistance-change type oxygen sensor having the structure wherein noncombusted components become attached to the surface of the sensor, it is found that poor reproducibility and low stability disables practical application.